That's dreadful - the fastest thing is to get on to the local paper. If it's deep in the country then this'll be a weekly publication. E-mail the news desk and paste IronMan's image into the message. They'll want a photograph of someone glum stood by it. Bombard the local contact at E.H. with messages until an officer 'makes a site inspection'. It's not going to fall over for a couple of weeks yet. The DEFRA officer that deals with the farm can also put great pressure to bear, if you can find him, but it's not unlikely that it was he that made a grant to create the pond, in the first place. Dorset Wildlife Trust will know who he is.
Even crap ponds like this are desperately needed in the country. Most - maybe 80% - have been destroyed in the last fifty years and the wildlife associated with them - there's none shown in the pictures of the Hellstones - is, in many cases, in a freefall to oblivion. Looking at the stone walls in the pictures it is clear that the farmer doesn't maintain his walls - they're falling down. So it's no surprise that the monument is going the same way. The stock will get a similar treatment so, please, make sure they don't suffer. What is the long term answer ? A license to farm ? I recall that the Dorset heathland species, particularly moths and butterflies, are going zilch. Species crash just as quickly as cars or space shuttles - but on a slower timeframe. Saving stones is peasy.
There'll be a lot of archaeology hidden in that wall - possibly 'portable stones'. It would't be impossible to do a more complete restoration on that monument. Put a mound back over the top of it eg. Some of the stones, in the wall behind, will have been the retaining wall around the entrance.
I'm having trouble with the Burnt Edge stone row in Bolton. I've made a 1024 x 768 and a 800 x 600 images of the aerial view, from MultiMap, and have been e-mailing these to various local 'officers'. What's this then? They generally pretend to be on holiday or somewhere else. Bolton's oldest graffiti. Et al. The Golf course has several heaps of debris scattered about - just of rubbish - and one of them has a very tall 'gatepost' in there - at about three metres. (The long stones are so easily broken !) I'm wondering whether the Dublin golf course has similar heaps - maybe containing bits of white stone ?